sheet piles, sand bags, or wait for the next drought and build a coffer dam and do it traditionally.
 
I believe that thousands of bricks were laid underwater when Winchester cathedral was underpinned.

sheet piles are the current favoured method.
Do you know the depth of water to contend with in order to reach the base of the wall and does this fluctuate with the tides .
We have done similar in the past with gabion baskets and even large precast concrete blocks laid in the water to build the new wall upon.

Not sure if you resolved this but more info about water depth, navigation restrictions, details and condition of the wall, etc, should be understood prior to coming up with options for repair. A consulting engineer specialising in water engineering will be able to help/advise. This is what I do but am a long way from Humberside!
 
There are expert consulting engineers who could help. Try googling Peter Brett Assoc who I have worked with, but you may be better off with someone more local as they are more midlands and southern based. You could ring your local Environment Agency office who will have a local area inspector who will be able to give the name of a local firm. A good engineer will be able to inspect the structure and come up with solutions to suit your budget.